The use of various ophthalmic devices, particularly those useful for application directly on the eye such as contact lenses, are known for the correction of vision difficulties. However, such devices have not been accepted by many people, in part because of the time often required by the eye to become accustomed to their use, the physiological intolerance that sometimes develops often after only short periods of use, or the inadequate visual correction which may be provided by them. Consequently, a great amount of effort has been expended in attempts to solve these problems for the contact lens wearer.
These efforts have included modification in both the design of the lens and the composition of the lens material. Modifications in lens design so as to improve the fit of the lens to the eye have produced only marginal reduction of the physiological intolerance in some people. Modification of the composition of the lens material has also only been partially successful.
Early contact lenses were made from polymethyl methacrylate (PMMA), a hard material that is not water-absorptive. Lenses of PMMA, although they are readily machinable and have excellent optical clarity and durability, have a high level of physiological intolerance for many individuals partly because of the stiffness of the lens and partly because the eye becomes deprived of oxygen due to the low oxygen permeability of the PMMA.
Lenses of soft water-absorptive hydrogel materials which bind free water, such as poly(2-hydroxyethyl methacrylate) or poly(vinylpyrrolidone), cause less irritation but at the cost of reduced visual acuity and greatly increased maintenance of the lenses. When such hydrogel lenses are hydrated, they also have a high oxygen permeability and generally are, therefore, comfortable to wear. However, the hydrated lenses have poor durability and exhibit a tendency to become cloudy due to adsorption of proteinaceous and lipid materials.
Lenses of silicone rubber are easily molded, are said to possess a softness similar to that of the upper lid of the human eye and have a high permeability to oxygen. However, contact lenses of silicone rubber may cause a burning sensation in the eye because of the low thermal conductivity of silicone rubber. Also, such lenses have been reported to sometimes tighten over the cornea in a "suction cup" fashion that impedes movement of the lens. This permits entrapment of toxic products and debris under the lenses and prevents tears from washing away this debris and lubricating the eyes, thereby causing abrasion of the eye. Furthermore, silicone rubber, being lipophilic, mucophilic, and inherently non-wettable, attracts debris, e.g., proteins, lipids, mucoids, and the like.
Ophthalmic devices such as contact lenses have also been fabricated from polysiloxane derivatives. U.S. Pat. No. 4,136,250 discloses a water-insoluble hydrophilic gel suitable for contact lenses that is a copolymer of 10 to 80 percent polyolefinic polysiloxane monomers and 90 to 20 percent hydrophilic monomers. The polyolefinic polysiloxane monomers are prepared, in one embodiment, by reaction of a polysiloxane polyol with a diisocyanate followed by reaction with a hydroxyalkyl acrylate. By such means, polyolefinic polysiloxane monomers are obtained which have in each ethylenically-unsaturated group two urethane groups.
U.S. Pat. No. 4,277,595 teaches shaped articles including contact lenses that are fabricated from a copolymer of acrylic acid and poly(organosiloxane) terminated by an activated unsaturated group, such as methacryloxy, acryloxy, or acrylamido. However, this reference also discloses that polysiloxanes having diurethane linkages, i.e., ##STR1## are undesirable for use in medical devices, particularly contact lenses (see col. 2, lines 57-67). It is implied that such a diurethane linkage imparts hydrolytic instability to the lens, i.e., it is degraded by water.
Further, U.S. Pat. No. 4,259,467, which discloses contact lenses made from polymers of polysiloxane having pendent hydrophilic groups, e.g. polyoxyalkylene, amido, and the like, states in col. 2, lines 39-47, that a polysiloxane incorporating a monourethane linkage, i.e., ##STR2## is likewise undesirable for use in medical devices, particularly contact lenses.
U.S. Pat. No. 3,808,178 discloses hard contact lenses that are prepared from a copolymer of a low molecular weight polysiloxanylalkyl methacrylate and an alkyl acrylate that can be made wettable to some degree by the incorporation of 0.1 to 10 percent by weight of one or more hydrophilic monomers. Such contact lenses have low oxygen permeability.
U.S. Pat. No. 4,261,875 teaches hydrophilic contact lenses made from polymers of a polysiloxane having a plurality of pendent hydroxyalkyl groups and at least one ethylenically-unsaturated group.